This invention relates to a gas cutting method and a gas cutting apparatus adapted to effectively cut stainless steel or stainless clad steel, for example.
For cutting of stainless steel and stainless-clad steel (hereinafter, referred to simply as "stainless steel"), there has been no way available except for the flux-oxygen cutting method and the power cutting method. This is because, in oxygen cutting, it is necessary to expose the cut surface of the material to the air in order to allow the reaction (combustion) of the cut surface of the material with oxygen gas. However, when the stainless steel is fused, slag (oxides) which has a melting temperature higher than that of stainless steel and is hard to fuse is produced. This slag covers the cut surface of the stainless steel thereby hindering combustion of the stainless steel and making the cutting impossible.
In powder cutting and flux cutting, iron powder or antioxidizing flux is supplied into the oxygen stream from a cutting torch and is burnt along with the stainless steel to heighten the temperature of combustion and to enhance the flowability of the slag with the result that the slag is scattered by virtue of the jet of oxygen from the torch. However, in these conventional cutting methods, well-dried superfine particles of iron or flux have to be sprayed constantly and uniformly into the oxygen stream at a high speed. This requirement has necessitated sophisticated techniques and made powder cutting devices extremely complicated in structure, giving rise to mechanical malfunctions and inadequate operation. Also, the conventional powder cutting and flux cutting devices have not been effective in underwater operations because powder or flux becomes wet and sticks inside hole of the torch and consequently, the oxygen stream from the torch is disrupted.
In order to eliminate the foregoing disadvantage, the inventor previously proposed a cutting torch according to the called wire cutting method. In this device, as illustrated in FIG. 1, preheating gas G (acetylene, propane, hydrogen, etc.) is injected from the tip of a cutting torch 1 toward a material M to be cut to form a preheating flame 2 and oxygen O.sub.2 is injected to form an oxygen stream 3, as in the common gas cutting device. The material preheated by the flame 2 is burnt by the oxygen, but refractory slag produced when stainless steel is cut cannot be removed for reason mentioned earlier. Then, a fine steel wire W playing the role of the iron powder in powder cutting is supplied into the oxygen stream 3 and is burnt to heighten the combustion temperature with the result that the slag is fused and scattered. This method is very effective in cutting a relatively thin plate of stainless steel. However, it is difficult to cut a stainless steel plate with a thickness of 50 mm or more because the wire supplied into the oxygen stream interferes therewith.
In passing, it is noted that the inventor has developed another cutting torch capable of cutting thick metallic plate (U.S. Pat. No. 3,967,090). The art described in U.S. Pat. No. 4,273,982 is similar to the above U.S. Patent. The devices taught by these arts are respectively provided with a water jet nozzle to the rear of the gas cutting torch. This water jet nozzle is adapted to inject pressure water for blowing off the dross (slag) produced when cutting a metallic material by use of the cutting torch. However, these devices are improper for cutting stainless steel insomuch as the slag produced from the stainless steel cannot be blown off and besides, the water injected from the nozzle in the proximity of the cutting torch reduces cutting efficiency.
Otherwise, an arc cutting methods such as metal arc cutting and oxygen arc cutting are applied for cutting metals including stainless steel. However, the electrode used in an arc cutting device is so quickly consumed that it is not suitable for continuous cutting over long periods as in the case of cutting a thick and large stainless steel plate.
Because of the situation described above, a new cutting technique adapted to efficiently cut thick stainless steel plates continuously over long periods even under water and being applicable to an automatic system has been desired.